Solar tracking system

ABSTRACT

A solar energy conversion apparatus which includes a reflective parabolic trough which tracks the sun the control means therefor being such that the trough rotates about a north-south axis alternately to east and west extreme positions until the driving motor is disabled by equal irradiation of a pair of phototransistors. The phototransistors are aligned relative to the axis of the trough such that they are equally irradiated when the sun&#39;s rays are concentrated on a heat transfer tube extending along the axis of the trough. The apparatus has a flexible disc drive member to mechanically couple the trough to the drive motor. The heat transfer conduit is of relatively small capacity and a two-phase flow therein is utilized so as to conserve energy.

This is a division of application Ser. No. 698,893, filed Feb. 6, 1985,now U.S. Pat. No. 4,649,900 issued Mar. 17, 1987.

This invention relates to solar energy systems.

This invention relates to a solar tracking system, more particularly,the invention is concerned with improvements in and relating to solartracking systems for use with solar tracking troughs which are arrangedto concentrate the sun's rays on a heat transfer target.

According to the first aspect of the invention there is provided amethod of constraining an object to track the sun, the object beingmounted for rotation about an axis and having a reversible motor forrotating the object and a control circuit for controlling operation ofthe motor, the control circuit having first and second limit switcheswhich are operable to reverse the direction of rotation when the objecthas reached first and second extreme positions, said method comprisingthe steps of:

(a) orientating the object so that its axis lies in predetermineddirection,

(b) activating the control circuit so that the motor causes rotation ofthe object,

(c) generating an interrupt signal to interrupt supply of power to themotor when the object is directed at the sun, and

(d) after generation of the interrupt signal, re-applying power to themotor such that it rotates the object towards the westerly directionuntil generation of a subsequent interrupt signal on realignment of theobject with the sun.

Preferably the interrupt signal is generated in response to equalirradiation by the sun of a pair of solar radiation sensitive elements.

According to a second aspect of the invention there is provided solartracking apparatus comprising an object mounted for rotation about anaxis which, in use, is oriented in a predetermined direction, areversible motor coupled to cause rotation of the object about said axisand control means for controlling the motor such that it rotates theobject between first and second extreme positions which, in use, arelocated towards the West and East respectively, said control meansincluding solar energy sensitive elements mounted for movement with theobject and operable to produce an interrupt signal when the object isaligned with the sun to arrest movement of the object, said interruptsignal inhibiting rotation of the motor whilst the object remainsaligned with the sun and wherein the control circuit causes rotation ofthe object towards said first extreme position when the object is nolonger aligned with the sun.

Preferably the predetermined direction is the North-South direction.

The solar energy sensitive elements are preferably connected torespective inputs of a differential amplifier and arranged to producethe interrupt signal when the phototransistors are equally irradiated bysolar radiation.

The method of the invention preferably includes the further step ofhaving a predetermined delay say of 10 minutes each time the first andsecond limit switches are activated. This prevents repetitive forwardand reverse movement during those periods when there is insufficientsolar intensity to generate the interrupt signal.

The invention will now be further described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic view of a solar collecting apparatus incorporatingthe tracking system of the invention;

FIG. 2 is a cross sectional view of a tracking head of the invention;

FIG. 3 is a side view of the head;

FIG. 4 is a fragmentary view taken along the line 4--4;

FIG. 5 is a schematic illustration showing the east and west limitswitches;

FIG. 6 shows a preferred circuit for use in the system of the invention;

FIG. 7 shows diagrammatically one technique for coupling the drive motorto the reflective trough;

FIG. 8 is a side view of part of the arrangement shown in FIG. 7;

FIG. 9 diagrammatically illustrates part of an improved reflectivetrough structure;

FIG. 10 is a similar view of a modified form of structure;

FIG. 11 is a schematic view of a solar conversion system of theinvention;

FIG. 12 is a more detailed view of one part of the system shown in FIG.1; and

FIG. 13 is a schematic view of a heat transfer fluid circulation systemof the invention.

The solar tracking apparatus shown in FIG. 1 comprises a parabolictrough 2 mounted for rotation about its axis 4 on a framework 6 havinguprights 8. A heat transfer tube 10 is located along the axis of thetrough and when the trough is correctly aligned at the sun, solar energyis concentrated onto the heat transfer tube 10. The trough 2 includes acounterbalance bar 12 carried by arms 14 connected to end plates 16 ofthe trough. The bar 12 carries a tracking head 18 which generatessignals for causing alignment of the trough with the sun. East and westlimit switches 20 and 22 are mounted in a housing 24 which is connectedto one of the uprights 8. The axis 4 is oriented in the north-southdirection. When the trough 2 is rotated to its extreme position towardsthe east the arm 14 engages a pivoted lever 26 which in turn operatesthe microswitch 20, as seen in FIG. 5. When the trough is rotatedtowards its western extremity, the arm 14 will engage the pivoted arm 26from the opposite direction and cause operation of the other microswitch22 as shown in broken lines in FIG. 5. As will be described hereinafter,the switches 20 and 22 cause reversal of the direction of rotation ofthe trough after elapse of a predetermined period. When the troughrotates through a position in which it is aligned with the sun that isto say when all of the sun's rays are focused on its focal line 4, thetracking head 18 will produce an interrupt signal to arrest movement ofthe trough and the trough will remain stationary with the sun's raysconcentrated on the heat transfer tube 10. Subsequent movement of thesun's rays relative to the head 18 will cause the head 18 to generatefurther tracking signals to cause the trough to follow the sun andmaintain the sun's rays concentrated on the transfer tube 10.

In the preferred form of apparatus, the east and west limit switches 20and 22 are located within the housing 24 connected to the upright 8. Thesame housing can be used for other control circuitry of the system. Thearm 26 extends through an opening in the housing and its inner end isconnected to the opposite wall of the housing by means of a tensionspring 28. Inward movement of the arm 26 is limited by a collar 30mounted on the arm 26 and engagable with the housing 24.

The preferred form of tracking head 18 is illustrated in FIGS. 2, 3 and4. The head includes a housing 32 having an elongate narrow slot 34 inits top surface 36 which is surmounted by a glass pane 35, the housingthus being closed. Mounted generally centrally of the base 38 of thehousing, is a platform 40 on the upper surface of which is mounted twophototransistors 42 and 44 or other solar energy sensitive cells. In useof the head 18, the slot 34 is arranged to be parallel to the axis 4 ofthe trough and thus the slot 34 extends in the north-south direction.The lateral position of the platform 40 relative to the housing 32 canbe adjusted by means of an adjusting screw 46 which extends through aopening in the housing 32 and is received within a nut (not shown)connected to the platform 40. A compression spring 48 encircles the stemof the screw 46 and acts between the housing 32 and the platform 40 andprevents unwanted movement of the platform. When the head 18 iscorrectly aligned with the sun, the sun's rays 50 will impinge upon thephototransistors 42 and 44 to an equal extent, as diagrammaticallyillustrated in FIG. 2. The adjusting screw 46 can be adjusted so as toensure that the concentrated solar radiation from the trough 2 isfocused on the heat transfer tube 10 when equal irradiation of thephototransistors 42 and 44 occurs.

The preferred form of control circuitry is illustrated in FIG. 6. Thecircuit includes a power supply 52 for producing DC levels required forthe circuit and for a motor 54 which rotates the trough 2 about its axis4.

The circuit includes a relay coil 56 having contacts 58 for reversingthe motor 54 when the east or west limit switches 20 or 22 are engaged.When the relay coil 56 is not energised the contacts 58 are such thatthe motor 54 rotates the trough 2 towards the west. The circuit alsoincludes a master cell for enabling operation of the circuitry duringdaylight hours only. In the illustrated circuit, the master cellcomprises a phototransistor 60. The other inputs to the circuitry arethe phototransistors 42 and 44.

The circuit further includes a timer 62 which is clocked by pulses fromoscillator 63 whose pulse rate is say 3 to 4 Hz. The timer 62 operatesto produce a predetermined delay, say 10 minutes, after the east or westlimit switch 20 or 22 has been activated so as to prevent unwantedcontinuous reversing of the trough 2 when there is enough daylight toactivate the cell 60 but insufficient intensity to cause the trackingphototransistors 42 and 44 to operate.

On applying power to the supply circuit 52, the Q output of a flip-flop64 will be high and that will remain high until the east limit switch 20is activated. The Q output of the flip-flop 64 is connected to a OR gate66 the output of which is coupled to the base of a relay drivetransistor 68 via a resistor 70 and capacitor 72. When the Q output ofthe flip-flop 64 is high the drive transistor 68 will be on and the coil56 will operate the contacts 58 so that the motor 54 will be rotatingsuch that the trough 2 rotates towards the east. The resistor 70 andcapacitor 72 provide a time delay of the order of say 200 millisecondsto ensure that the motor 54 has been arrested as will be explainedhereinafter prior to operation of the contacts 58. The Q output of theflip-flop 64 is coupled to an input of OR gate 94 which enables power tobe supplied to the motor 54 as will be explained hereinafter.

On reaching the easterly limit, the switch 20 is closed causing a rapiddischarge of capacitor 74. The capacitor 74 is coupled to the resetinput R of the flip-flop 64 via an inverting amplifier 76 so thatdischarge of the capacitor 74 causes the Q output of the flip-flop 64 togo low. This, in turn, to causes the relay coil drive transistor 68 toturn off and hence the relay contacts 58 to reverse. Additionally, theoutput of the amplifier 76 is coupled to AND gate 120 the output ofwhich is coupled to timer 62 as explained hereinafter. This couplingcauses power to be cut to the motor 54 for a period determined by thetimer 62, the arrangement being such that the motor is arrested beforereversal of the contacts 58 by virtue of the time delay caused by theresistor 70 and capacitor 72 which are coupled to the base electrode ofthe transistor 68.

The output of the amplifier 76 is also coupled to a differentiatingcircuit 78 to produce, on closing of switch 20 a positive pulse which isapplied to the reset terminal R of the timer 62 via OR gates 80, 82 and84. The output from the timer 62 is connected to the set input of aflip-flop 86 which functions as a storage device to hold the output ofthe timer 62. The reset terminal R of the flip-flop 86 is coupled to theoutput of OR gate 82 and is thus reset on closing of the east limitswitch 20. Output from the flip-flop 86 is connected to the baseelectrode of a motor drive FET 88 via OR gates 90, 92 and 94, AND gate96, and two inverters 98 and 100. The other input to the AND gate 96,line 102, is high when the phototransistors 42 and 44 are not equallyirradiated so that a high level on the Q output of the flip-flop 86 willcause the FET 88 to turn on and thus the motor 54 to rotate in adirection as determined by the contacts 58. Thus, when the east limitswitch 20 is closed the timer 62 and flip-flop 86 are reset, the Qoutput of the flip-flop 86 will be low untill 10 minutes have elapsed asdetermined by the timer 62 whereupon the Q output will then go high andturn on the FET 88 and so cause operation of the motor 54.

Assuming that the phototransistors 42 and 44 do not produce signalsindicative of correct alignment of the apparatus with the sun, the levelon the line 102 will remain high and the motor 54 will continue torotate the trough towards the west until the west limit switch 22 isclosed. This will cause capacitor 104 to discharge thereby causing theoutput of inverting amplifier 106 to go high. The output of theamplifier 106 is connected to the set input of a flip-flop 108 the Qoutput of which is connected to an input of OR gate 80 via adifferentiating circuit 110 and to an input of OR gate 66. Closing ofthe west limit switch 22 will therefore cause resetting of the timer 62and flip-flop 86. The Q output of the flip-flop 86 will go low and theFET 88 will remain off until the timer 62 turns it on again. Closing ofthe west limit switch 22 will activate the relay coil 56 to reverse thecontacts 58 after the time delay caused by the resistance 70 andcapacitor 72 to place the contacts 58 in a position appropriate foreastward rotation of the trough 2. The output of amplifier 76 is coupledto the Reset terminal of flip-flop 108 via OR gate 146 to reset theflip-flop on closure of the easterly limit switch 20.

The motor 54 is arranged to have dynamic braking and this is affected bycoupling of a 0.220 ohm impedance 112 across the motor terminals whenthe FET 88 is to be turned off. This is accomplished by coupling theoutput of the inverting amplifier 98 to transistors 114 and 116, thetransistor 116 being turned on to couple the impedance 112 across themotor terminals to thus absorb the energy in the motor quickly.

After closure of the switch 22, the trough will remain at the westerlylimit for ten minutes as determined by the timer 62 and thereafter theFET 88 will be turned on to cause the motor 54 to again rotate thetrough towards the east. The repetitive rotation reversals and delayswill thus continue until the phototransistors 42 and 44 cause a changein the level on line 102. Further, this mode could be interrupted bydeactivation of the daylight sensing phototransistor 60. If thetransistor 60 turns off, the output of inverting amplifier 118 will gohigh. The output of amplifier 118 is connected to one input of an ANDgate 120 the other input of which is connected to the output ofamplifier 76. When the trough rotates to its easterly extreme andactivates the switch 20, the output of the gate 120 will go high thusmaintaining a high level on the Reset input of the timer 62. This willbe maintained whilst the daylight sensing transistor 60 is off thuspreventing turning on of the FET 88. Normally the transistor 60 will gooff in the evening and the trough is therefore conveniently orientedtowards the east for resumption of tracking the following morning.

The circuit also includes a fault detector which senses when the motor54 has been running for longer than a predetermined period which islonger than the time it takes to rotate the trough from the easternlimit to the western limit. That rotation time is typically of the orderof say one or two minutes and so the fault period is set at say fiveminutes. If the motor is sensed as running longer than the fault period,it is assumed that the trough has been jammed and accordingly the motoris deactivated. This is effected by providing a timing circuit 122clocked by the oscillator 63, the Reset terminal being coupled to theoutput of inverting amplifier 98. The circuit 122 is thus reset afterthe delay caused by the timer 62 has elapsed on activation of the limitswitches 20 and 22. The Q output of the timer 122 is connected to theinput of an amplifier 124 the output of which is connected to the baseelectrode of a transistor 126. The collector of the transistor isconnected to the gate electrode of the FET 88 the arrangement being suchthat when the Q output of the timer 122 is low the transistor 126 is offand it does not affect the operation of the FET 88 as determined by theoutput of the inverting amplifier 100. If however the timer 122 is notreset before say five minutes its Q output goes high causing thetransistor 126 to turn on thus holding the gate electrode of the FET 88at a low level whereupon it cannot be turned on and thus the motor willnot run.

The circuit includes a sensor for sensing excessively high currents inthe motor and arranging to turn the FET 88 off when overcurrents aresensed. This is effected by placing a resistor 128 (0.010 ohms) inseries with the FET 88 and coupling the voltage level at the resistor128 to the input of amplifier 124 so that if the motor current whichflows through the FET 88 exceeds a certain level, the higher voltage onresistor 128 will cause the transistor 126 to turn on thus holding theFET 88 off.

The emitters of the phototransistors 42 and 44 are connected to inputsof a quad analogue switch 130 which comprises an intergrated circuitserial No. 4066, which functions as a reversing switch controlled by theQ and Q outputs of the flip-flop 108. The outputs of the switch 130 areconnected to the inputs of a differential amplifier 132 the output ofwhich is in turn connected to the input of a Schmitt trigger 134 and theinto one input of an NAND gate 136. The emitters of the phototransistors42 and 44 are also connected to a gate 138 which functions as an OR gate(In the illustrated circuit, three NAND gates are connected together soas to logically function as an OR gate). Output of the gate 138 isconnected to the other input of the NAND gate 136. The arrangement issuch that the output of the gate 138 will be high when one or both ofthe phototransistors 42 and 44 are on and low when they are both off.This enables the gate 136 to distinguish between the cases where (a) thedifferential amplifier 132 has a zero output which will occur when bothphototransistors 42 and 44 are irradiated to the same degree and (b)when neither is irradiated at all. Output from the NAND gate 136 iscoupled via inverting amplifiers 140 and 142 to the line 102 into theAND gate 96. Thus, when the phototransistors 42 and 44 are irradiatedequally, there will be zero output from the differential amplifier 32and the trigger 134 will produce a high pulse which triggers the NANDgate 136. This will produce a low pulse on the line 102 and thus causethe output of AND gate 96 to go low and hence the FET 88 will turn off.

Output from the NAND gate 136 is coupled by an inverting amplifier 144to the Reset terminal of the flip-flop 108 via OR gate 146 so as toreset that flip-flop causing its Q output to go high. If the trough istracking in the westerly direction, the Q output of the flip-flop 108will already be high there will be no change. If however the trough istracking in the easterly direction, the Q output will be low and will bechanged to high by the pulse applied to its Reset terminal by the gate146, and hence the relay coil 56 will be operated so as to change thecontacts 58 so that thereafter the trough will track in the westerlydirection to follow the sun.

Output from the gate 138 is applied via an inverting amplifier 148 to aninput of OR gate 82 so as to reset the timer 62 via OR gate 84 and toreset the flip-flop 86. This causes the FET 88 to remain off for tenminutes as determined by the timer 62 each time the output of the gate138 goes low corresponding to loss of sufficient intensity of solarradiation to cause the phototransistors 42 and 44 to remain on. Forinstance, if the sun is obliterated by a dark cloud, the trough willremain motionless for ten minutes and after that period the FET 88 willbe turned on and the trough will be rotated in the westerly directionuntil the phototransistors 42 and 44 again produce equal output.

In order to avoid tracking inaccuracies owing to different positions ofthe head 18 in accordance with whether focus is achieved whilst trackingin the easterly direction on the one hand and the westerly direction onthe other, the reversing switch 130 is used so as to switch the outputsof the phototransistors 42 and 44 into the inputs of the differentialamplifier 132 in accordance with the direction of rotation. This isaccomplished by coupling the switch 130 to the outputs of the flip-flop108 the outputs of which are dependant upon the direction of rotation.

The output of gate 138, which senses when at least one of thephototransistors 42 and 44 is on, is applied through amplifier 148,delay circuit 150 and amplifier 152 to the base electrode of a drivetransistor 154. The transistor 154 controls the flow of current to apump relay 156 the contacts 158 of which control supply of power to apump 160 which pumps heat transfer fluid through the tube 10. The delaycircuit 150 serves to keep the transistor 154 on for a predeterminedperiod, say two minutes, after both of the phototransistors 42 and 44have been turned off through loss of sun. This prevents undesirableswitching of the pump when the sun's rays are partially obscured byclouds and also ensures that solar energy concentrated onto the tube 10has sufficient time to be transferred to the heat transfer fluid flowingwithin the tube 10.

When both of the phototransistors 42 and 44 are off, the output of gate138 will be low and accordingly the output of inverting amplifier 148will be high. The output of amplifier 148 is applied to the Resetterminal of timer 62 through OR gates 82 and 84 and to the Resetterminal of flip-flop 86 through OR gate 82. The arrangement is suchthat after the elapse of ten minutes without either of thephototransistors 42 and 44 being turned on, the Q output of flip-flop 86will go high and turn the FET 88 on so that it will cause rotation ofthe trough with reversals and delays as described previously. If howeverthe sun is only momentarily lost, the output from the gate 138 is madeto override the resetting of the timer 62 to prevent the trough enteringinto the reversal and delay mode. This is accomplished by coupling theoutput of the gate 138 to the OR gate 92, the output of the gate 92being high when either of the phototransistors 42 and 44 is on. Thiseffectively nullifies the effect of resetting of the timer 62 and givescontrol of switching of the FET 88 to the line 102 which is coupled tothe phototransistors 42 and 44 through the differential amplifier 132.

The circuit may include a manually operated switch 162 which on closingapplies a high input to the OR gate 90 so as to provide for switching onof the FET 88 manually so as to rotate the trough to a selected positionfor parking, washing or maintenance.

FIGS. 7 and 8 diagrammatically illustrate one technique for coupling themotor 54 to the trough 2. The arrangement is an improvement of thearrangement illustrated in Australian Patent Application No. 29159/84.In the illustrated arrangement, the motor 54 is mounted upon a baseplate 176 which is carried by a relatively flexible mounting plate 178.The output shaft of the motor 54 is connected to a gear box 180 theoutput of which is connected to a conical friction drive member 182. Thedrive member 182 is in engagement with a sheet metal drive disc 184which is coupled with the trough 2 so that rotation of the disc 184causes rotation of the trough. The arrangement also includes a conicalreaction disc 186 of relatively large diameter, the disc 186 beingresiliently biased into engagement with the drive disc 184 so that thefriction disc 82 maintains good driving contact with the outer peripheryof the driving disc 84. The disc 186 is mounted for rotation on a shaft187 carried by a bracket 189 which has one leg 191 in pivotal contactwith a flange 183 formed on the plate 176. A shaft 195 extends from theflange 193 through an opening (not shown) in the bracket 189 and aspring 197 acts between the shaft 195 and the bracket 183 to bias thelatter towards the drive member 182. Axes 188 and 190 of the drivemember and disc 186 intersect with the plane of the drive disc 184 atthe axis 4 of the trough so as to maintain rolling contact between themember 182, drive disc 184 and reaction disc 186. Both the mountingplate 178 and the drive disc 184 are made from relatively thin sheetmaterial such as stainless steel so that these parts can undergo flexureabout axes perpendicular to the axis 4 but are relatively inflexiblerelative to axes parallel to the axis 4. In other words, the disc 184and plate 178 do not flex in the direction in which driving forces aretransmitted but are capable of flexure in other directions so as toaccommodate manufacturing imperfections and mis-alignments. Ifnecessary, the drive disc 184 may be provided with circumferentiallyextending slots 192 as is illustrated in FIG. 9 to increase its flexure.

FIG. 9 illustrates part of a reflective parabolic trough madeanalogously with the general arrangement disclosed in U.S. Pat. No.4,321,909. The trough includes a plurality of ribs 252 each of which hasan accurately formed concave parabolic edge 254. Reflective aluminiumsheets 256 are biased into intimate contact with the edges 254 so thatthe sheets 256 form a reflective parabolic surface. The lateral edges ofthe sheets 256 are formed with U-shaped channels 258 which have the dualpurpose of serving as a mounting recess for the edges of glass panes 260and to strengthen the lateral edges of the sheets 256 to preventunwanted deflections. In the arrangement illustrated in FIG. 9, thesheets 256 are biased into intimate contact with the edges 254 by meansof compression springs 262 which act between the upper face of thechannel 258 and a lever 264 which is pivoted to the ribs 252 at 266. Theother end of the lever 264 has nut 268 connected to or in engagementtherewith, the nuts receiving an adjusting screw 270 formed on the rib.The arrangement is such that adjustment of the screw 270 alters thecompressive force supplied to the spring 262 and hence to the sheets256.

The arrangement illustrated in FIG. 10 is somewhat analogous except thatthe spring 262 and lever 264 are omitted and adjusting screw 274 isdirectly upon the upper face of the channel member 258.

Both the arrangements of FIGS. 9 and 10 are relatively unaffected bydifferential thermal expansions between the ribs 252 and the sheets 256.

The solar energy utilization system shown in FIGS. 11 and 12 comprises aplurality of tracking solar collecting troughs 2 which concentrate solarenergy on to heat transfer conduits 10. For instance, the troughs 2 maybe tracking parabolic troughs of the type disclosed in U.S. Pat. No.4,321,909. The conduits 10 are connected to a common line 306 whichleads to a solar energy utilization apparatus 308 which functions as aheat transfer element and a condenser for any vaporized heat transferfluid in the common line 306. A fluid return line 318 returns heattransfer fluid from the apparatus 308 to the tops of the troughs 2 viaan expansion column 310, pump 312, filter 314, and capillary tubes 316.The expansion column 310, pump 312, and filter 314 are connectedgenerally speaking in the return line 318 and the capillary tubes 316branch off from the return line to the respective heat transfer conduits10 of the troughs 2. The pump 312 has a by-pass valve 320 connectedacross it, the valve 320 serving to regulate the pressure of the heattransfer fluid at the outlet of the pump.

The expansion column 310 includes a chamber 322 which has an inlet port324 located near its top and connected to the return line 318 from theapparatus 8. The chamber has an outlet port 326 located at a lower levelthan the port 324 and connected to the return line 318 leading to thepump 312. The arrangement is such that the outlet port 326 will act as atrap for air bubbles which will rise through a third port 328 which isconnected to a conduit 330 which leads to a header tank 332. The headertank 332 serves as a reservoir for replenishing heat transfer fluid andis connected to a supply line 334 via a float valve 336. The conduit 330includes a pressure relief valve 338 which will permit heat transferfluid to pass through the conduit 330 into the header tank 332 whenexpansion of the fluid causes the pressure to rise above the openingpoint of the valve 338. When the temperature of the system falls andthermal contraction occurs, a check valve 340 connected across thepressure relief valve 338 will permit flow of heat transfer fluid fromthe header tank 332 into the conduit 330 and then into the return line318. The presence of the header tank 332 connected in the system in themanner shown always ensures that a positive pressure is maintained inthe heat transfer system.

The system could operate with various fluids as the heat transfer fluid.A particularly suitable heat transfer fluid would be water and theoperation of the system will be described with water as an example. In atypical arrangement, the pump 312 could be arranged to produce apressure of about 100 p.s.i. at its output, there being no significantpressure drop along the return line 318 until the start of the capillarytubes 316. There will be a considerable pressure drop across thecapillary tubes 316 say of the order of 70 p.s.i., the actual pressuredrop depending on flow rate. The pressure drop in each of the capillarytubes can be made substantially uniform. There will be a relativelyinsignificant pressure drop along the heat transfer conduits 10 andabout 30 p.s.i. pressure drop along the common line 306 and through theapparatus 308, whereby minor differences in pressure drops which mayoccur in the various conduits 10 are swamped out.

The flow rate through the respective heat transfer conduits 10 isarranged to be such that when the troughs 2 are receiving maximum solarradiation the flow rate through each of them is chosen so that it willvaporize approximately 50 percent of the water supplied thereto. Forcollectors which have a collecting surface of approximately 10 squaremeters, a flow rate of about 4.5 liters per hour is all that isnecessary. The lengths and diameters of the capillary tubes 316 areselected so as to give the required pressure drop at the flow ratethrough the respective heat transfer conduits 10. For a flow rate of 4.5liters per hour a capillary tubing which has an internal bore of say 2mmand a length of about 50 feet will give the required 70 p.s.i. pressuredrop.

When the system is operating, the flow rates through the collectors issuch that approximately half of the water supplied thereto is vaporizedand a significant proportion of the energy transferred through thecommon lines 306 will be in the form of latent heat of vaporization ofthe steam, that latent heat being made available in the apparatus 308 oncondensation of the steam.

It has been calculated that for a system where there are 20 collectorseach of approximately 10 square meters collecting surface the returnline 306 and the line 318 may comprise copper tubing or pipe of anominal diameter of 0.5 inches. This is considerably less than thediameter required in known systems and consequently the losses from thelines 306 and 318 are very small.

The power supply to the pump 320 is controlled by thermostatic elements(not shown) located in the heat transfer conduits 10 the arrangementbeing such that the pump 312 is only stopped when all of thethermostatic elements register temperatures below predetermined levelsso that flow will continue even though only one of the collectors isproducing useful output. The flow through the other collectors is sosmall that it does not significantly affect the overall performance eventhough the design temperature is not reached in the heat transfer fluid.It is further preferred that the pump 312 continues for a predeterminedperiod say 5 minutes after each of the thermostatic elements has fallenbelow operating temperature so as to avoid unnecessary starting andstopping of the pump. Once however the pump does stop, any vapour in thereturn lines 306 will be condensed in the apparatus 308 and thus itsenergy will be usefully exchanged in that apparatus. This compares mostfavourably with single phase systems in which no such transfer wouldoccur.

FIG. 13 schematically illustrates a system for circulating a heattransfer fluid. In the system, a heat transfer fluid such as water isstored in a header tank 342 which is connected to the upper end of theheat transfer conduit 10 via a check valve 346. Heat energy is appliedto the conduit 10 at a temperature which is sufficient to causevaporization of the heat transfer fluid. This may be accomplished bymeans of a tracking parabolic trough 2 which concentrates the sun's raysupon the conduit 10. The other end of the conduit 10 is connected to oneend of a heat transfer coil 350 located within a hot water storagevessel 352. The other end of the coil 350 is connected to the headertank 342 via a check valve 354.

When sufficient heat is applied to the conduit 10 in order to raise thetemperature of the water to boiling point, vaporization will occur atleast at one zone in the conduit 10. This will be accompanied by anincrease in pressure which will force the water and steam towards thecoil 350 since it cannot flow in the reverse direction because of thecheck valve 346. The hot water will thus flow through the coil 350 andthereby transfer heat to the water within the hot water vessel 352 andthen to the header tank 342 via the check valve 354. The stage will bereached when there will be only steam remaining in the conduit 344 andcoil 350 whereupon the vessel 352 and coil 350 will serve as acondenser, condensing the steam to the liquid phase which will beaccompanied by a sudden drop in pressure. Water cannot flow from theheader tank 342 to the coil 350 because of the check valve 354 but watercan flow through the check valve 346 into the conduit 344 and coil 350.The collector 348 will once again heat the water within the conduit 344so that circulation will occur when boiling point is reached. It will beappreciated that this process will be repeated as long as the trough 2can supply sufficient energy to the conduit in order to induce boiling.

From the point of view of energy transfer, it will be appreciated thatthe solar energy collected by the trough 2 is transferred to the waterwithin the vessel 342 without the use of any external pumps. It isfurther noted that the vessel 352 can be located at a lower levelrelative to the collector 348 since the system does not rely onconvection currents. This is seen as a significant advantage because thecollector could be mounted on a roof of a house or other building andthe vessel 352 located at ground level.

I claim:
 1. A solar energy utilization system comprising a plurality ofsolar energy collecting devices each having a heat transfer conduit fora heat transfer fluid and solar energy concentrating means forconentrating solar energy on said conduit so as to heat the heattransfer fluid sufficiently to cause vaporization of the fluid to amixed liquid and vapor phase, heat utilization means which, in use,condenses heat transfer fluid supplied thereto in the vapor phase, aheat transfer fluid circulating system for circulating heat transferfluid from the devices to the heat utilization means, wherein saidcirculating system includes a first line including a pump for supplyingthe heat transfer fluid in liquid form at a predetermined pressure tosaid devices, a second line for conveying heat transfer fluid in saidmixed liquid and vapor phase from said devices to the heat utilizationmeans, said heat transfer conduits being connected in parallel betweensaid first and second lines, flow control means for each of saiddevices, said flow control means being connected in series with saidheat transfer conduits for controlling flow of the heat transfer fluidso as to render the flow through respective heat transfer conduitssubstantially independent of one another;each flow control meanscomprising a flow element operative, in use, to cause a relatively largepressure drop across it so as to substantially eliminate the effects ofvariations in pressure drop across the various collectors or variationsin pressure caused by vaporization of the heat transfer fluid in thecollectors; the flow element comprising capillary tubing.
 2. A systemaccording to claim 1, wherein the solar energy concentrating meanscomprises a reflective parabolic trough which is arranged to track thesun.
 3. A system according to claim 1, including a header tank and acondenser connected therewith wherein the heat transfer fluid compriseswater which, in use, is partially vaporized in the concentrator and theincreased pressure forces the water and steam to pass through thecondenser into the header tank.